1. Field of the Invention
The invention relates to a light emitting assembly, more particularly to a light emitting assembly including a light emitting package and a heat sink having a mesa protruding therefrom through a circuit board to be bonded to the light emitting package.
2. Description of the Related Art
Development of light emitting diode (LED) packages in the LED industry since 1970 to the present day can be divided into successive generations in terms of thermal resistance. The LED packages from the early generation to the latest generation include 5 mm LED package, Low Profile LED package, Low Profile with extended lead frame LED package, Heat-sink Slug LED package (see FIG. 1), and SMD (Surface Mount Device) Ceramic LED package (see FIG. 2), which have thermal resistances of 250 K/W, 125 K/W, 75 K/W, 15 K/W, and 6 K/W, respectively. Early developed LED packages have a relatively low brightness and are normally used in applications, such as indicators in personal computers, electronic devices, and the like. These LED packages have characteristics of low power consumption and low heat generation. In the indicator applications, since the heat generated by the LED packages is relatively small and since their installed quantity in a unit area is small, they are allowed to be mounted directly on a printed circuit board (which serves as a power connecting medium to permit power to be transmitted to the LED packages mounted thereon) without considering how the heat generated by the LED packages can be dissipated. However, when the LED packages are to be used in illuminating applications, such as vehicle headlights and projector lamps, they are required to be high power LED packages and to have a high density so as to achieve a high luminance and brightness, which results in high heat generation. As such, the heat generated by the high power LED packages cannot be neglected, and the heat dissipating issue becomes relatively important to the service life of the high power LED packages. As illustrated in FIG. 1, the aforementioned heat-sink slug LED package (a non-surface mountable device) normally includes a light emitting die 16 enclosed by a dome-shaped encapsulant 14, a molding material 19 molded over leadframe leads 15 of the light emitting die 16, and a heat-sink slug 17 of a thermally conductive material embedded in the molding material 19 and bonded to the light emitting die 16 for enhancing heat dissipation. U.S. Pat. No. 6,274,924 discloses a heat-sink slug LED package of this type. Referring further to FIG. 2, the aforementioned SMD ceramic LED package differs from the heat-sink slug LED package in that the former further includes a ceramic layer 18 bonded to the heat-sink slug 17 for permitting the LED package to be surface mountable and for enhancing heat dissipation. Although the LED packages of FIG. 1 and FIG. 2 have an improved heat dissipation, the heat dissipation issue is still a major problem for these LED packages when used in illuminating applications, particularly for applications, such as vehicle headlights and projectors, that require a high density of the LED packages and that can provide light focusing function. Since the LED packages to be used for illuminating applications are high power LED packages, fast transfer of the heat generated by the high power LED packages to the outside environment is crucial. As such, mounting of the high power LED packages directly on the printed circuit board in a conventional manner as those of the low power LED packages is not feasible due to the thermal insulating property of the dielectric substrate of the printed circuit board, which is made from an insulating material, such as phenolic cotton paper, and which is a thermal barrier for transferring the heat from the LED packages to the outside environment. In order to solve this problem, metal core PCBs have been developed in recent years to replace the conventional printed circuit board for serving as a power connecting medium for the high power LED packages mounted thereon as well as to act as a heat sink to dissipate heat from the LED packages which the conventional printed circuit board fails to provide. Referring back to FIGS. 1 and 2, the metal core PCB includes an anodized aluminum substrate 11 with an anodized surface 13, an insulator layer 12 formed on the anodized surface 13 of the aluminum substrate 11, and conductive traces 112 formed on the insulator layer 12. When the LED packages of FIG. 1 are mounted on the metal core PCB, the leadframe leads 15 are bonded to the conductive traces 112, and the heat-sink slug 17 is bonded to the insulator layer 12 through a thermally conductive adhesive so as to transfer heat from the LED package to the outside environment through the aluminum substrate 11. When the LED packages of FIG. 2 are mounted on the metal core PCB, solder bumps 151 on the ceramic layer 18 are bonded to the conductive traces 112, and the ceramic layer 18 is bonded to the insulator layer 12 through a thermally conductive adhesive 113 so as to transfer heat from the LED package to the outside environment through the aluminum substrate 11. Note that since the standard requirement of the voltage resistance for a substrate on which the LED packages are to be mounted is 2.5 kV and since the voltage resistance of the anodized surface 13 of the aluminum substrate 11 is about 220 to 400V, the insulator layer 12 on the aluminum substrate 11 is required so as to meet the requirement. Although the aluminum substrate 11 has a heat conduction coefficient of 225 w/mk, the insulator layer 12 is made from an epoxy material and the heat conduction coefficient of the insulator layer 12 is relatively low. As a consequence, the heat dissipation rate of the metal core PCB is considerably and adversely affected by the presence of the insulator layer 12, and is still insufficient to permit use of the LED packages in high brightness and/or high light focusing (which means a high density of LED packages) illuminating applications. Current solutions to the heat dissipating problem of the LED packages in the illuminating applications are focused on how to improve the heat dissipation of the metal core PCB. However, the heat dissipation rate of the metal core PCB is considerably limited by the insulating layer 12.